Processing facilities

Once uranium ore has been mined from an ore body, it requires additional processing to concentrate it and remove impurities to then produce the final product. In general, uranium processing facilities contain many circuits, each of which has its own specific radiation concerns. These can vary greatly depending on many factors, such as age, the expected composition of the uranium ore to be processed, and environmental constraints. However, there are general features which are common to all. Figure 13 provides a simplified flow chart of a uranium extraction process while Fig. 14 provides a more specific flow chart for the McClean Lake mill in Canada [39].

6.6.1.1. Ore handling and preparation

Ore blending may be necessary when a uniform grade is required for processing. In anticipation of processing, ore is often stockpiled on a pad, where it can be mixed, as needed. Uranium ore is mechanically reduced in size to increase the efficiency of extraction, and the processes can involve crushing, ball mills, rod mills, or autogenous or semi-autogenous grinding processes. In cases where slurry is created directly from an underground mine, or after the grinding process has been completed, material is pumped into storage tanks and held in preparation for blending and leaching. Slurry density is often adjusted in thickener tanks to prepare it for processing.

6.6.1.2. Uranium leaching into solution

Ore slurry is transferred from storage vessels to the extraction circuit.

Uranium is leached from ore using either acid or alkali leaching processes, largely depending on the geochemical properties of the ore. Leaching vessels are typically connected in series, with one tank feeding another. Chemical reagents are mixed into each tank, as needed.

6.6.1.3. Classification and liquid–solid separation

After extraction, material is transferred into a series of tanks designed to separate the uranium bearing solution from the remaining solids and extract any residual uranium. Solid material is directed towards the tailings circuit, while the uranium bearing solution is directed towards the concentration and purification circuit.

FIG. 13. A simplified uranium processing flow chart (see www.chemcases.

com/nuclear/nc-06.htm).

FIG. 14. Specific schematic of the processing (courtesy of D. Chambers, SENES Consultants).

6.6.1.4. Concentration and purification

To remove any remaining suspended solids, the uranium bearing solution is passed through a collection of tanks and filters. Once filtered, the solution is sent to the extraction circuit, while the solids are returned for further separation.

The uranium bearing solution undergoes a series of chemical transformations to produce a more concentrated solution. Common techniques include solvent extraction and ion exchange. Once most of the impurities have been removed, the solution is sent for precipitation, where the uranium is chemically changed into a solid. After the uranium has been precipitated it is allowed to settle out in solid form. The solid material is sent to a thickener tank to increase its density. This solid uranium compound is then sent to a drying circuit.

6.6.1.5. Drying and packaging

The final step in the process is drying and packaging. Excess liquid is removed prior to drying by using filters or centrifuges. Drying is performed with either a low temperature dryer or a high temperature calciner. The chemical composition of the final product depends on the temperature at which it was dried. In a low temperature dryer, solid uranium compounds are dried at around 300°C and transformed into a soluble uranium final product. In a high temperature calciner, solid uranium is dried at around 800°C and transformed into an insoluble uranium final product. Once the uranium product has been dried, the product is packaged into drums for storage and transport. Ideally, the product packaging stage is carried out using an automated, ventilated drum filler to reduce occupational exposures.

6.6.1.6. Final product storage and shipping

Uranium final product drums awaiting shipment need to be stored in a secure, clean area with restricted access. A typical storage arrangement could be a separate building or shipping containers packed and awaiting final shipment.

Owing to the ingrowth of decay products, uranium product stored for an extended period will gradually emit increasing amounts of gamma radiation.

6.6.1.7. Tailings preparation and storage

Waste streams from every part of the process are concentrated and treated in tailings preparation tanks. There are typically multiple treatments due to the variety of materials in the waste streams. Once treated, the tailings are thickened and deposited for long term storage. Treated tailings need to be stored in a

manner that reduces the potential for negative consequences to the environment and the general population in the long term. They are often stored in specifically constructed cells, mined out surface pits or mined out underground drifts in such a way that they remain isolated, contained and chemically inert.

6.6.1.8. Water treatment

Water used during uranium processing usually requires treatment before release into the environment. Part of the treatment process can involve the removal of radioisotopes and other pollutants through precipitation. In the case of radium removal, the potential for RDP generation and localized gamma dose rates needs to be addressed in the monitoring programme.

6.6.2. Design and operation

The design and operation of a processing facility depend on local site specific factors. Factors such as the grade of the ore, the type of mineralization, the availability of reagents, the availability of waste disposal facilities and the volume of ore to be processed are critical in the design of the facility. External factors such as the topography of the region, the weather, water availability, the amount of land available and nearby public populations can also influence the design of the facility.

Most uranium processing plants are designed for the most efficient extraction. This generally means that the distances between plant sections and the amount of crossing over between the sections are minimized. Sometimes local topography is used to allow gravity to assist in the flow of material within the process. The ability to change or expand the process is a critical design decision and plants have experienced significant difficulties by not considering this in the design phase. Consideration of maintenance needs is also critical during the design phase and aspects such as access for heavy lift cranes need to be assessed.

As uranium operations can be in remote areas, associated infrastructure such as power, water and transport are also critical.

Consideration of radiological aspects during design is not always a priority for the design engineers. Historically, this has led to increased occupational doses during either operations or maintenance. During design, radiation exposure may be one of the key considerations, depending on the ore grade and processing techniques. For example, an aspects to be considered is the proximity of large volumes of ore or waste to personnel (e.g. in stockpiles, processing tanks, tailings storage). The ore stockpiles are often located adjacent to the processing plant for efficiency, and this can result in increased occupational exposures.

Potential exposures can be greatly reduced at the design stage by proper consideration of engineered controls. As most of the processing plant is wet, ease of cleanup and containment of spills can offer the dual benefit of lower doses and more efficient plants. For areas where there is higher potential for exposure, such as during uranium final product packaging, additional engineered controls can be incorporated, such as negative pressure rooms and automated drum filling and cleaning.

Processing uranium ore is normally relatively easy to control and as the material is contained within the plant, occupational exposures are generally stable and well defined. However, unexpected sources of exposure can occur that were not considered in the process design or when the plant does not operate as designed. These can arise from changes in the plant (e.g. leaking pipes or changed ventilation), changes in personnel practices and changes in the ore composition, as a result of changes in reagents or as the result of material accumulation over time (e.g. scale in pipes and tanks). The impact can be significant for occupational doses if they are not detected by the monitoring programme. Maintenance is also a critical consideration for occupational exposure at uranium processing facilities.

Activities involving vessel entries, scale removal, tank cleanout, ventilation repair and baghouse filter changes have a high potential for occupational exposure. Ideally, both the plant and the task will have been designed with dose minimization in mind.

An important radiological consideration in the design and operation of the processing facility is potential disequilibrium across the circuit. For most plants, there is extremely good information on the behaviour of uranium (²³⁸U, ²³⁴U, ²³⁵U), since uranium is the target ore and it is relatively easy to measure using conventional chemical instruments. However, the behaviour and disposition of the other radionuclides (²³⁰Th, ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po) are far less well known and can change the potential for occupational exposure. The general approach is that the material can be considered to be in equilibrium prior to the addition of water in the grinding phase. After this point, the various radionuclides in the uranium series behave according to their chemical and physical properties.

After leaching, it can be assumed that the liquor has enhanced uranium and the solids have reduced uranium, and that only uranium isotopes are present in the final product recovery area. Care needs to be taken with this general approach;

performing a radionuclide balance on the process is the best way of ensuring that the behaviour of all radionuclides is understood.

6.6.3. Principal exposure pathways

For most processing plant workers, the principal exposure pathway is external gamma radiation because most processing is in the form of a liquid

or saturated solid and the material is fully contained in the process equipment.

In these cases, the exposure from inhalation of both long lived alpha emitters in airborne dust and radon progeny is negligible. However, there are many exceptions, such as initial ore crushing, cleanup of spills, maintenance activities and activity in the product packaging area. Radon gas is released during the processing of the ore and if there is insufficient ventilation (either natural or forced), radon progeny exposure can become significant.

The amount of gamma exposure in a facility will depend on the uranium ore grade, the quantity of ore and the proximity of ore to the general work areas.

Beta radiation can also pose a hazard in a processing facility, particularly to the eyes. Ore stockpiles and tanks are often the largest source of gamma radiation.

Over time, tank liners can become entrained with gamma emitting radionuclides, which cannot be easily removed. Material buildup in back end processing tanks and recycled process water piping can result in steadily increasing gamma radiation exposure rates due to the ingrowth of uranium decay products. Even when emptied and flushed, tanks can be significant sources of gamma radiation, and the dose rate will depend on the size of the tank and the ore grade being processed. Any work done to repair tanks or tank liners needs to be closely managed, with gamma radiation monitoring, including EPDs, where appropriate.

Initial ore crushing is generally performed on relatively dry material, so dust is likely to be generated. Control mechanisms such as water sprays, ventilation and the exclusion of personnel from the area can reduce this potential pathway.

The cleanup of spilt material can give rise to enhanced gamma exposure and has the potential for dust generation and subsequent inhalation. Where possible, wet cleaning methods, such as hosing into sumps, are to be used. Another area of cleaning is the collection of accumulated dry material, such as spilt material from conveyors and accumulated material around deteriorating tanks and piping.

Cleanup of this material may need to take place in dry conditions, so PPE precautions against inhalation may be warranted.

During maintenance activities the inhalation of both radon progeny and dust needs to be considered. Enclosed vessels or areas can have elevated radon gas levels and a good standard practice is to perform a radon or radon progeny measurement before any vessel or confined space entry. Radon exposure also needs to be considered in other areas of poor ventilation, such as reclaim tunnels under ore stockpiles. A good practice is to flush with clean air prior to personnel entry. Inhalation of radionuclides in airborne dust is possible when dry operations are being performed. Water based cleanup is to be used, where possible. The potential for inhalation of dust needs to be considered for any maintenance work which is either in inherently dusty areas (e.g. ventilation system maintenance, filter changes) or directly generates dust (cutting, grinding).

In most processing plants, the highest potential exposure is associated with the inhalation of uranium isotopes in airborne dust within the final product packaging area. This is due to a combination of the material being dry and hence able to become airborne and the far higher specific activity compared to elsewhere in the processing plant. Special monitoring and control mechanisms are usually put in place in the packaging area to control this potential dose. The greatest risk of internal exposure occurs when process material is released from containment. In the precipitation and solvent extraction circuits, material is more readily inhaled or ingested after it has been dissolved into water used for cleaning.

Mist escaping from tank hatches can also contain soluble radioactive material.

6.6.4. Control mechanisms

Ideally, radioactive material is contained to minimize exposure to the general workforce. This is especially true when large quantities of material are being stored or transferred. Secondary containment such as enclosures or bunded or bermed areas is extremely effective at isolating material from the general work areas when primary containment fails or has to undergo maintenance.

In a processing facility, gamma radiation is often considered to be the primary radiation hazard. Significant quantities of product need to be stored away from personnel, and any work done in a storage areas minimized. If gamma radiation dose rates are expected to be higher than workplace objectives, placing tanks within well shielded enclosures needs to be considered. If work takes place near large sources of gamma radiation for an extended period, shielding can reduce gamma dose rates to acceptable workplace levels.

Slurry or tailings transfer pipes need to be located a reasonable distance from general walkways and work areas. When this is not feasible, localized gamma radiation can be controlled by wrapping pipes with a shielding material, such as lead. Care needs to be taken to ensure that piping can withstand the increased weight of any added shielding.

Well designed ventilation systems may be needed to control worker exposure to airborne radiation hazards. Where there is a high potential for exposure, storage and processing tanks can be ventilated individually and the air exhausted away from working areas. If possible, the tank process exhaust will have redundant fans to maintain ventilation in case of a primary fan failure. The tanks can be negatively pressurized to prevent radon and radon progeny from escaping in the event that the tank hatches are opened for inspection or sampling.

Buildings which house the majority of the processing equipment can benefit from a single-pass ventilation strategy, such that clean air is loaded with contaminants of increasing concentration before being exhausted. In buildings with multiple floors, air needs to be drawn down to lower floors before being

exhausted, as radon will naturally collect in low lying areas. It is especially important that the building exhaust location is carefully selected so that the ventilation intakes are unlikely to recirculate contaminated air and to ensure that outdoor work areas are not in the exhaust stream.

Offices and control rooms are generally considered to be clean areas, free of radioactive contamination, and need to be isolated from the main processes of a facility. When this not possible (e.g. control rooms are usually in, or close to, the work area), routine workplace cleaning of floors, tables, desks and computers can help to reduce exposure. Care is needed to ensure that contaminated PPE is cleaned or removed before entering or working in an office or control room.

When offices or control rooms are located near gamma radiation sources, shielding can ensure that dose rates are within workplace objectives. Positive pressure ventilation can ensure that offices and control rooms are kept free of airborne radiation hazards.

Although ventilation is less of a concern in open air facilities, it is important to identify the location of all source terms with respect to regular working areas. In the case of open air ore and slurry storage, it is possible to experience larger than expected concentrations of radon and radon progeny under calm weather conditions.

A wide range of administrative controls can minimize doses within a processing facility, including the use of safe working levels, controlled and supervised areas, and restrictions on access and the consumption of food and water. An appropriate level of training for all staff and feedback to them on the radiation levels in work areas can prove very effective at reducing doses. This training can include detailed information about the specific radiation risks in an area, particularly for higher potential exposure tasks and areas such as final product packaging and maintenance operations. External factors also have to be considered, such as the impact of weather: rain, temperature inversions, still winds and hot and cold spells can all effect radiation exposure.

A key aspect of the facility waste management programme is to develop a system whereby contaminated items are collected and stored separately from clean items. Special bins for contaminated waste need to be readily available wherever waste is generated. In certain locations, waste disposal may be frequented by wildlife foraging for food, so any edible waste can be isolated from contaminated waste to prevent the ingestion of radionuclides by wildlife.

Incinerating food waste may need to be considered if segregation is ineffective.

A uranium operation is expected to consider all potential accident situations when developing an emergency response plan. In most operations, the potential radiation exposure is not significant enough to prevent emergency responders from performing their duties. This is not necessarily the case in very high grade uranium processing operations and a more thorough analysis is necessary. Some

accidents can also result in significant contamination with radioactive liquids and slurries, which has to be addressed in the emergency response plan, along with the appropriate protective measures.

6.6.4.1. External exposure

External exposure to gamma radiation is controlled by time, distance and shielding. The time spent near localized gamma sources needs to be minimized or the work needs to be performed at a distance from significant gamma sources.

Fixed work stations ought to be in low dose rate areas. The use of modern EPDs can be very effective in controlling and reducing gamma doses.

Fixed work stations ought to be in low dose rate areas. The use of modern EPDs can be very effective in controlling and reducing gamma doses.